The purpose of this work is to modify the genomes of the mRNA viruses BMV (brome mosaic virus), Q beta bacteriophage and SBMV (southern bean mosaic virus) and their relatives in order to study the fundamental processes of viral genome replication, viral protein synthesis, virion assembly and their interrelationships. The immediate goals in making these modifications are both to alter virus-host interactions and to enable the construction of vectors for the introduction of new or alternative viral functions. Constructions will be made in which viral cDNAs and their RNA transcripts are intermediates in the infectious process in order to use the powerful techniques of recombinant DNA chemistry. Modifications will be introduced to further identify the proteins encoded in viral genomes and delineate their mechanism of action and to identify and characterize genomic regulatory sites.